A Brief Scientific Introduction to an Egg

Eggs: a Very Very Brief History…

In the kitchen a few types of eggs are readily available; goose, duck, quail but none more so than the chicken. Which leads to the age old question - which came first the chicken or the egg?  A biblical solution to this problem can come from Genesis, where it is said that the father created all the creatures not their embryotic subunits. From a more accurate standpoint there can be only one answer; the egg. Eggs pre-date the earliest chickens whose origins can be found as jungle fowls that existed in South East Asia or India over four thousand years ago. Eggs not only pre date chickens, they pre date birds and are one of the earliest forms of sexual reproduction dating back over 1 billion years!

Egg Ageing

The egg is unique to the cook in that it is the second alkaline source in the kitchen, the first being bicarbonate of soda. The alkaline quality only increases with the age of the egg but to a detrimental effect of the produce. The increased alkalinity of the egg over time is due to the porous nature of the egg shell.

Porous Egg Shell (Scanning Electron Microscope) SEM Image 

We all know that storing eggs within the fridge can lead to the eggs taking on “fridge smell” of whatever is currently being stored in there. This can be used to the cooks’ advantage, for example if eggs are stored in an air tight container with a truffle (the mushroom), the resulting eggs will be perfumed with truffle. Digression aside carbon dioxide gas exits the egg over time; it is this gas that provides the acidic element to an egg and once removed the egg becomes more alkaline. Carbon dioxide is an important acid that our own body must deal with on a day to day basis. When we respire (produce usable energy) we produce carbon dioxide as a side product and when dissolved in a liquid, like water, it increases the liquids acidity. We use homeostasis to prevent our blood from becoming too acidic / alkaline. The egg however has no homeostatic ability therefore the yolk increases from a pH of 6 to 6.6 and the white from 7.7 to 9 and above. While these numbers seem like small changes we must remember that pH is on a log scale therefore a change from 7.7 to 9 is around a 20 fold increase in alkalinity.

Why does a rise in pH lead to a poorer quality egg? Proteins called albumens are spherical balls that are made up of a string of carefully folded amino acids. This ball has negative charges on its surface and similar to magnets, like charges repel one another. In doing so the proteins are kept apart from one another. This property is increased as the egg ages and the pH becomes more basic through the alkaline egg liquid amplifying the negative charge. The stronger the charge the more watery the egg will become due to the proteins not being in close proximity to each other and rubbing past one another.

A Cartoon Displaying how the Intermolecular Distance Between Proteins Changes with Age, Salt and pH  

A further effect of egg ageing is where the yolk becomes more fragile over time. The white contains a higher water percentage which over time diffuses into the yolk, increasing the yolks volume causing it to swell and making it more likely to burst the thin membrane containing the yolk.

Cooking and Culinary Uses

There are three major culinary uses of eggs other than just for the joy of eating them. These include: the stabilisation of emulsions such as mayonnaise; the thickening of liquids into gels and semisolids (things likes custards take advantage of this property); and finally eggs are used to create light textures in cakes etc. through the use of foams. Here we will examine an egg’s ability to set and thicken other liquids and we will focus on emulsions and foams another day.

Other than the obvious high water content, we have briefly talked about how proteins are a major component within eggs and how they determine the structural properties of the egg. Proteins are long chains of amino acid bricks making up a long sequence  / chain. As discussed these chains fold into specific shapes than can for all intents and purposes be thought of as a ball. This ball shape needs to be unfolded in order to cook the egg. Luckily for the cook there are a few ways we can do this; either through the addition of heat, acid or salt. The most common practice of cooking eggs is though the addition of heat. Heat provides energy which breaks apart the internal bonds holding the folded protein ball shape together. Once the weak internal bonds are broken, stronger bonds between proteins can form linking different albumin proteins together. When proteins begin to bind together they trap water within a net structure and this net structure provides a solid texture to the cooked egg, which reflects light making egg whites…well white. The texture of the egg can still be modified through further cooking; additional heat energy allows more protein protein bonding to occur which squeezes out the trapped water; resulting in a firmer texture.

A Cartoon Displaying Three Changes Proteins go Through when Cooked

This brings us nicely to curdling. Curdling is the process where proteins in a net structure separate out from a liquid such as the water in custard. Curdling is caused by proteins bonding very tightly with themselves, squeezing out so much water that it becomes a solid bit of egg protein floating in a liquid.

The addition of other ingredients heavily dictates the temperature the eggs will cook at. Through adding acid (citrus / vinegars) we are adding additional hydrogen atoms which have a positive charge. These small positive charges screen the negative charges (opposites attract) on the surface of the spherical folded proteins. Through reducing this negative charge proteins can come closer together decreasing the time and energy required for unfolding and bonding with other proteins. It is worth noting that pH will also cause some of the protein to unfold. Salt has a similar approach to speeding up the cooking of eggs; the salt when dissolved in water dissociates with itself on a molecular level into its composite charged ions; sodium and chorine. The positive charge of sodium has the same effect as seen with the acidic hydrogen ions.

As cooks we rarely just cook an egg with salt and acid, we add other more delicious ingredients such as cream, milk and sugar. Milk and cream have a high water content and dilute down the proteins so they are less likely to interact with one another. In order to cook and set the egg milk mixture the temperature has to be higher to result in proteins moving  fast enough to raise the possibility of one protein meeting another and bonding together. Sugar on the other hand has a strange effect of coating the proteins with thousands of little sucrose molecules thereby preventing protein interactions and reactions, again raising the cooking temperature.

Cooking Technique

We are informed as cooks that boiling an egg is the simplest form of cooking that we could ever achieve but how is that so? I would say that cooking the bird is easier than its embryotic beginnings. Roasting a bird at 180 ^oC for some length of time will result in it turning brown; once brown all over and when you pull at its legs and they fall off, you know it’s cooked. An egg on the other hand has no reference point to know when it’s cooked or what stage of “cooked” it is at. If you want hard boiled eggs well that’s easy enough, boil the egg for ten minutes. But what if you want that illusive runny yolk? No matter what size the chicken is the reference points stay the same, however the size of the egg drastically changes the cooking time. The problem is that boiling water is too hot with the exception of boiling an egg on top of Everest (where the decrease in pressure on the water’s surface leads to a reduced boiling temperature – however in these conditions you might be better having a cereal bar than a boiled egg). At 100 ° C (roughly 300 kelvin) all the proteins in the egg unfold and link up setting both the white and the yolk. When we cook a boiled egg is a race against time to select the exact point at which enough energy has diffused through the egg to set the white but not so far to ensuring that the yolk remains runny - this sort of cooking just requires practice. A better way of cooking an egg is through varying the temperature. So far we have discussed protein binding as a whole rather than individual protein types but I imagine by now that when I say different proteins have different cooking temperatures you will not be astounded. These different cooking temperatures can be used to the cooks’ advantage when “boiling” an egg. If we select at temperature at which the white is cooked but the yolk is still runny,  we will have cracked it….I’ve done well to say that this is the only egg pun so far. Below is a figure not by me but robbed from (http://www.douglasbaldwin.com/sous-vide.html) showing the temperature at which an egg was cooked at vs its “cookedness”.

Temperature vs Egg "Cookedness"

From the image we can see how egg yolks change with increased temperature to a more firm consistency. However the whites are still not the perfect texture and this is what is referred to as an inverted egg: runny whites and set yolk. Years of research has been conducted on the egg which demonstrates that to cook a perfect egg we must blanch eggs at 95 -100 ° C  for 2 – 3 minutes to set the white, then the egg needs to be cooked at a more moderate 62.2 ° C for an hour to cook the yolk. I told you it was easier to cook the chicken.

As a final note on boiling eggs there is the matter of egg smell and discolouration. When boiling an egg sulphur atoms are released from the albumin proteins in the egg whites. These free sulphur atoms then pick up hydrogen atoms which form hydrogen sulfide gas (H₂S_(g)) which is the smell of cooked eggs. Over cooking results in more gas and a stronger egg smell. A rotten egg smell on the other hand is the result of the same gas but in a much higher quantity. Hydrogen sulphide is also responsible for the greyish colour around the yolk. As the egg is heated, pressure increases in the egg resulting in gas moving closer to the centre of the egg, which reacts with the iron present within the yolk making iron sulfide - the grey part surrounding the yolk of hard boiled eggs. Should you worry about overcooking your eggs? Yes hydrogen sulfide in very high concentrations can cause respiratory distress, nausea and death. It’s probably why rotten eggs smell so bad to us, as our evolutionally background makes sure we avoid them.

Poaching eggs for some is a method best avoided but with a bit of science knowhow success could be just lurking around the corner. To the water we could add acid in the form of vinegar which will promote cooking through helping to null the negative charge of proteins. In turn the frontier layer of proteins will coagulate quickly aiding the egg to stay in a more spherical shape – too much acid will affect the taste of the cooked egg. Salt could be added to the simmering water which again will help coagulate the outer layer of egg as previously discussed. However the main influencing factor in the quality of the poached egg is the quality of the egg to begin with. Unfortunately not all eggs are born equal and these are sorted into three types of egg quality: AA, A and B. AA eggs have the highest concentration of protein which is detected through a process called candling, whereas A and B eggs have less protein respectively. The higher concentrations of protein mean the egg white is thicker / viscus; meaning the egg while being poached, is less likely to float and fall apart during cooking. Also, as previously discussed, age is an important factor in determining egg viscosity; the older the egg the more difficult it is to poach.

Omelettes and scrambled eggs are made using milk, cream and butter however the cook must be careful as to the volume of liquid added. Too much liquid will dilute down the proteins so much so that they cannot interact and bond with each other to form a set texture. I have read that it is advised that the addition of 2-5 tsp of liquid is optimal loading per egg for scrambled eggs. Omelettes which form a stiffer coagulation will only take 2 – 3 tsp of added liquid. Butter can play a vital role in the overall texture of these products preventing a tough consistency forming. The butter fat prevents lots of bonds between the protein forming yielding a more enjoyable mouth feel and eating experience.

Custards, whether sweet or savoury, are egg gels. Curdling is your nemesis in these grounds so always cook over a gentle heat and resist the temptation to increase the temperature too high. Many a chef and cook has fallen victim to overheating – just remember that all you are trying to do by heating is to gently remove water bringing proteins and added fat globules closer in proximity to one another, and breaking the weak bond within individual proteins. Overheating curdles the egg forming strong bonds between the proteins that cannot be broken.

BOOM! I think that’s enough about eggs for one day but I do hope that you find this as interesting as I did. I will post more about egg science (specifically on how eggs help us form edible emulsions and foams) at a later date. Normal recipe blogging will resume first though.

References:

H. Mcgee, On Food and Cooking, Scribners, New York, 1^st ed. 1984

http://blog.khymos.org/2009/04/09/towards-the-perfect-soft-boiled-egg/ (accessed 24/2/15)

http://www.douglasbaldwin.com/sous-vide.html (accessed 24/2/15)

http://www.chefsteps.com/activities/the-egg-calculator?utm_source=twitter&utm_medium=post-the-egg-calculator-tw&utm_campaign=EggCalculatorCampaign (accessed 24/2/15)

http://avogadro.chem.iastate.edu/MSDS/hydrogen_sulfide.pdf (accessed 25/2/15)